The research proposed here derives from an unexpected convergence of two areas of current cancer research. One area describes the fact that carcinoma cells exploit certain cell-biological programs that are usually operative in normal embryonic development and function to convert epithelial cells to cells that have a more mesenchymal (connective tissue-like) appearance and behavior. This conversion is often termed an epithelial-mesenchymal transition (EMT) and, when appropriated by carcinoma cells, serves to impart to these cells critical powers, such as invasiveness and motility, both of which are essential for cancer cell metastasis. A second research area has addressed the existence of small minorities of self-renewing cancer cells that exist amid far larger numbers of non-self-renewing cancer cells within tumors. The minority cells, often termed cancer stem cells (CSCs), are capable of seeding new tumors, while the cancer cells forming the majority of cells within a tumor lack this ability. These CSCs mirror the behavior of the stem cells in normal tissues, which also constitute small minorities of the total cell populations in such tissues and, because of their self-renewing abilities, are also responsible for the regenerative abilities of these normal tissues. Recent research has revealed that when normal mammary epithelial cells (MECs) are induced to undergo an EMT, they acquire the self-renewing powers of normal epithelial stem cells. Analogously, when tumor cells are forced to undergo an EMT, they acquire far greater tumor-initiating ability and thus are likely to be CSCs. The ramifications of these recent findings will be explored in the proposed research. For example, there is reason to believe that an EMT suffices to convey carcinoma cells from a primary tumor to a site of distant metastasis. Some of this research will examine whether the resulting disseminated cells, having experienced an EMT, now gain the ability to form macroscopic metastases. This research will also reveal whether CSCs arise preferentially from normal cells, or whether they can be formed equally efficiently from normal non-stem cells. In addition, genetically altered mice will be constructed whose cells fluoresce in the event that they activate one of the several transcription factors that are known to program the EMT. The cells of these mice will make it possible to screen for the cell-to-cell signals that induce epithelial cells to undergo the EMT and induce non-stem cells to become stem cells.